China's Yi Tang made the early running in the women's 100m freestyle heats as defending champion Britta Steffen struggled to make the top-16 semi-finals of the event on Wednesday.
Yi, the year's fifth-ranked swimmer on times, cruised into the night semis in 53.28secs.
She showed the way from Melanie Schlanger, a member of Australia's victorious 4x100m freestyle team, and Denmark's Jeanette Ottesen.
Dutch flyer Ranomi Kromowidjojo, the fastest swimmer this year, was fifth into the semi-finals with 53.66secs, but Germany's Steffen barely made it into the last 16.
Steffen, who beat Libby Trickett and Natali Coughlin in the Beijing final four years ago, put in 54.42secs to scramble into the top 16.
It has been a frustrating Olympics in the pool for Germany who have to medal in the competition.